Methods and apparatus are known for securing together, with a fused or welded joint, overlapping portions of a tensioned loop of thermoplastic strap encircling an article. One approach is to apply a heated member to the overlapping strap portions to effect a melting of at least the facing strap surfaces after which the strap portions are held together while the melted material resolidifies to form a welded joint. A related method effects the melting at the strap surfaces with ultrasonic energy.
Another, and very effective, process forms a welded joint by first compressing the overlapping strap portions together and then creating unidirectional or multidirectional bodily sliding frictional movement between the contacting strap surface regions to melt interface regions of the overlapping strap portions. The melted interface regions are allowed to solidify so as to bond the overlapping strap portions together in a weld or joint.
This frictional process, which can be generally described by the term friction-fusion welding, has proven to be especially effective with conventional thermoplastic strap materials such as nylon, polyester, polypropylene, and,the like. Such conventional strap is typically provided in widths ranging from 5 mm. to 9.525 mm. and in thicknesses ranging between about 0.254 mm. and about 0.889 mm.
With conventional thermoplastic strap having a thickness of between about 0.254 mm. and about 0.889 mm., the overlapping strap portions are typically bonded together in a friction-fusion weld to a thickness of between about 0.013 mm. and 0.051 mm. in each overlapping strap portion across the entire width of the strap. Typically, the length of the friction-fusion bond extends for about 10 mm. to about 35 mm. along the length of the overlapping strap portions.
Although conventional strap works well in a great many applications, the inventor of the present invention has determined that it would be highly desirable to provide, in some applications, strap that is considerably wider than conventional strap (e.g., two to eight times as wide) and that is considerably thinner (e.g., 0.08 mm.).
Such thin film wide strap may also be transparent and could advantageously be used in certain applications, including in the binding of a stack of newspapers or magazines. The relatively wide strap would reduce the pressure on the stack of newspapers or magazines, particularly at the corners, and would thereby have less of a tendency to damage the newspapers or magazines. Finally, a relatively thin, transparent, strap readily permits viewing of the portion of the article which is covered by the strap.
The inventor of the present invention has determined that welded strap joints having a conventional configuration are difficult to employ satisfactorily with thin film wide strap, For one thing, much more energy would be required to melt the entire surface areas of the overlapping wide strap portions in the selected joint region. Further, control of the thickness of the fused material in the thin strap would be difficult. Also, care must be taken to avoid unwanted penetration of one or both of the overlapping strap portions. In addition, methods for melting the full strap width surface portions with heated members would require that the overlapping strap portions be subsequently held together for commercially unacceptable long periods of time as the strap joint cools.
Even more problems are encountered with thin film strap fabricated from so-called "oriented" materials, such as, for example, strap comprising linear crystallizable polypropylene that has been worked into a thin film having planar molecular orientation of the macromolecular chains with a uniplanar, axial oriented crystalline structure through at least a major portion of the film thickness. An attempt to produce a conventional weld in such film strap across the full width of the strap may result in reduced weld strength and can reduce the strap strength at the weld since the strap orientation is destroyed in the fused region of the weld.
The inventor has determined that it would be desirable to provide a fused joint or weld in overlapping portions of such thin film wide strap wherein the overlapping strap portions retain a sufficient amount of tensile strength after formation of the joint to enable the strap to properly function in a tensioned loop around an article at conventional strapping tensions for the applications in which such thin film strap would be used. Such an improved joint should desirably accommodate various means for effecting its formation at commercially acceptable speeds. Preferably, such an improved joint should accommodate its rapid formation by friction-fusion techniques.